This invention relates to long range ordered alloys of the transition metals V, Fe, Ni, and Co, which have been improved by substituting small quantities of titanium and zirconium for like quantitites of V for improvement of mechanical properties and which have been further modified by addition of cerium and niobium to improve creep properties.
Ordered alloys are a unique class of metallic materials which form long range ordered crystal structures below their critical ordering temperature, T.sub.c. Ordered alloys offer potential advantages over conventional disordered alloys for high temperature structural applications. Superior performance can be traced to the relatively low atomic mobility and unique dislocation dynamics in ordered lattices. The strength of ordered alloys does not degrade very rapidly with increasing temperature. In many cases, the yield strength of ordered alloys shows an increase rather than a decrease with increasing temperature. Long range order produces stronger bonding and closer packing between atoms. The restricted atomic mobility generally results in slower diffusion processes and better creep resistance in ordered lattices.
The advantage of LRO alloys is their strength and stability in use environments at high temperature. LRO alloys can experience high temperatures below T.sub.c for an indefinite period without undergoing significant compositional or phase changes. However, there are disadvantages at temperatures above T.sub.c and also at low temperatures substantially below T.sub.c. Above T.sub.c the tensile strength is substantially reduced due to the disordering effect, and at lower temperatures the principal disadvantages have been their extreme brittleness and low ductility.
Developments have recently been made in the improvement of LRO alloys. Cobalt-based alloys with the nominal compositions (Co,Fe).sub.3 V and (Co,Fe,Ni).sub.3 V and high T.sub.c have been shown to significantly improve ductility, see U.S. Pat. No. 4,144,059, Liu et al, Mar. 13, 1979. However, these alloys are of limited use for nuclear applications due to the high neutron absorption cross section resulting from the cobalt content, and they are expensive due to the high cost of cobalt.
Consequently, improvements have been made by development of iron-based LRO alloys, minimizing the amount of needed cobalt, see U.S. Pat. No. 4,238,229, Liu et al, Dec. 9, 1980. It was surprising to find that an alloy containing zero or only a small amount of cobalt would demonstrate ordered structure in combination with excellent mechanical properties. These iron-based alloys showed highly desirable combinations of low neutron absorption cross section, high tensile strength, high yield strength, good tensile elongation, with no brittle phase formation at elevated temperatures. The disadvantage of the Fe-based alloys is their lower T.sub.c than Co-based alloys, thus the improved properties just described occur at lower temperatures than for the previously described Co-based alloys and their ductility decreases as T.sub.c is approached. The base alloys exhibit a tendancy toward grain boundary fracture and reduced ducility resulting from both grain boundary weakness and high flow stress near T.sub.c. An LRO alloy with improved mechanical and metallurgical properties at elevated temperatures was yet to be developed.
Then it was found that additions of titanium and zirconium to these cobalt- and iron-based LRO alloys even further improved the ductility of the alloys at elevated temperatures, see U.S. Pat. No. 4,410,371 Liu et al, Oct. 18, 1983,. Creep tests indicated that these elements substantially increase the rupture ductility and extend the rupture life of the LRO alloys. Titanium additions also reduce the tendency toward intergrannular fatigue resistance of LRO alloys. However, excessive amounts of titanium (and probably other Group IV-A elements) significantly increase the creep rate and lower the creep resistance of LRO alloys. It was desired to further improve the creep properties of these alloys and that is an object of this invention.